Size Effect And Multi-scale Analysis Of Concrete Short Columns Confined With Aramid FRP Jackets

Fiber reinforced polymer (FRP) composites have been applied widely in civil engineering for strengthening and repairing existing concrete structures, because of their higher strength-to-mass ratio, better tensile mechanical performance, excellent easily constructing function, and other merits. Previous studies have shown that externally wrapped FRP sheets can increase the strength, ductility, and energy dissipation of confined concrete columns. However, most of current studies were based on testing of small scale specimens, and suggested prediction models and design formulas (for strength, deformation, and stress-strain relationship curve) were lack of consideration on the possible size effect of FRP-confined concrete columns. This may cause deviations in research and un-safeties in design.Size effect of concrete is generally defined as the phenomena that the strength of concrete decreases as the specimen's size increases. The main cause for the size effect is fracture performances during the failure of concrete. Thus, a rational method for the size effect should be able to describe the fracture behaviors during the failure. As a heterogeneous material, macroscopic failure of concrete is resulted from microscopic damage, so the failure process is necessary to be studied by multi-scale methods. At present, multi-scale methods for concrete are insufficiency and on beginning phase, the multi-scale analysis for FRP-confined concrete columns is still blank.In response to these circumstances, in this dissertation, experimental and theoretical studies are carried out on concrete short columns confined with aramid FRP (AFRP) sheets. The main works are as follows:1) An experimental investigation, including 99 AFRP-confined concrete short columns and 36 unconfined columns, was implemented. All the specimens could be divided into two types:ones with circular cross-section, and the others with square cross-section. The main experimental parameters were AFRP's confinement ratio and specimens'size. The experimental results show that the specimens'size has significant effect on the strength of columns, partial effect on the stress-strain relationship curves, slight effect on the ductility and energy dissipation, and scarce effect on the failure mode, respectively.2) A factor analysis is adopted on the test data, to probe into the characteristics of the size effect of AFRP-confined concrete short columns. Depending on the test data and statistical results, prediction models for the strengths, deformations, and stress-strain relationship curve of columns are presented. The specimens'size is taken into account in the model for strength, as well as the interaction between the specimens'size and AFRP's confinement ratio. However, the specimens'size is not a parameter for the models for deformation. All the prediction models herein can agree well with the test data.3) Base on the theory of microstructure failure, failure modes are established and formulated for concrete short columns under monotonic axial compression. Multi-scale analytical models of the ultimate strength for unconfined concrete columns and AFRP-confined concrete columns are deduced. In addition, the influence due to the AFRP confinement is discussed on the ultimate strength of AFRP-confined columns, as well as the applicability of the analytical models. The analytical models can consider the size effect on the ultimate strength of columns, and their results agree well with the test data.4) A fractal-microplane model is proposed, it can consider the effect of element's size on properties of concrete material. A multi-scale finite element model is developed for the AFRP-confined concrete short columns, combining the fractal-microplane model and the nonlocal theory. This model can provide good predictions on the stress-strain relationship curves of the columns and rational simulations on the failure propagation.